The preparation of foamed polyvinyl chloride molding compositions or sheets using chemical or physical blowing agents or using extruders under gas pressure is part of the state of the art (see, for example, "Kunststoff-Taschenbuch" [Plastics Pocketbook] 20th edition, Carl Hanser Verlag 1977, pp. 277, 349; or DOS No. 2,944,068, Shin Etsu Chem.). In general, closed-cell foam materials are produced having densities of 40-800 g/l. These polyvinyl chloride foam materials have not become popular as thermal insulating materials despite their inherent flame retardance, because their weight per unit volume is either too high, or their manufacture is too expensive due to the consumption of blowing agents and the use of costly processing.
The disadvantage of the known methods consists in that
1. only 2-step or 3-step-processes are applied, PA1 2. one has to work under elevated pressure, PA1 3. the molding times are extremly high. (Ullmanns Encyklopadie der technischen Chemie, 4. ed., vol 20, p. 427-14 28; JP No. 53 064 276; EP No. 00 40 443)